For the safe final disposal of highly radioactive wastes, it is known to add glass formers to them and, by known methods, to melt a glass therefrom which, after solidification, can be stored in the form of monolithic glass blocks in an appropriate container. Furthermore, products of extra safety margin have been developed, such as aggregates of glass and metal, in which the highly radio-active glass in the form of particles, which may be of sizes between two and eight millimeters, is embedded in a metal matrix (W. Heimerl, Atomwirtschaft-Atomtechnik, 20 (1975) pp. 347-349). In other methods, the highly radioactive glass block of appropriate composition is subjected to a controlled devitrification by a suitable heat treatment; examples of the kind of glass ceramics which have been proposed for formation by this method are those of the celsian, perowskite, diopside and eucryptite type (A.K. De, B. Luckscheiter, W. Lutze, G. Malow, E. Schwiewer, S. Tymochowicz, Management of Radioactive Wastes from the Nuclear Fuel Cycle, IAEA, Vienna 1976, Vol. II, pp. 63-73).